Apochromat

Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths.

Anapochromat,orapochromatic lens(apo), is aphotographicor otherlensthat has better correction ofchromaticandspherical aberrationthan the much more commonachromatlenses.

The prefixapo-comes from the Greek prepositionἀπό-,meaning free from or away from.

Explanation

Chromatic aberrationis the phenomenon of different colors focusing at different distances from a lens. In photography, chromatic aberration produces soft overall images, and color fringing at high-contrast edges, like an edge between black and white.Astronomersface similar problems, particularly withtelescopesthat use lenses rather thanmirrors.Achromaticlenses are corrected to bringtwowavelengthsintofocusin the same plane – typically red (~0.590μm) and blue (~0.495μm).Apochromatic lenses are designed to bringthreecolors into focus in the same plane – typically red (~0.620μm), green (~0.530μm), and blue (~0.465μm).The residual color error (secondary spectrum) can be up to an order of magnitude less than for an achromatic lens of equivalent aperture and focal length. Apochromats are also corrected forspherical aberrationat two wavelengths, rather than one as in an achromat.

Apochromatic lens brings three colors to a common focal plane. Notice that this lens is designed for astronomy, not viewing, since one of the wavelengths (~0.780μm) is in thenear infrared,outside the visible spectrum.

Telescope objective lenses for wide-band digital imaging in astronomy must have apochromatic correction, as the optical sensitivity of typical CCD imaging arrays can extend from theultravioletthrough thevisible spectrumand into thenear infraredwavelength range. Apochromatic lenses for astrophotography in the 60–150 mm aperture range have been developed and marketed by several firms, withfocal ratiosranging fromf/5tof/7. Focused and guided properly during the exposure, these apochromatic objectives are capable of producing the sharpest wide-field astrophotographs optically possible for the given aperture sizes.

Graphic artsprocess (copy) cameras generally use apochromatic lenses for sharpest possible imagery as well. Classically designed apochromatic process camera lenses generally have a maximum aperture limited to aboutf/9. More recently, higher-speed apochromatic lenses have been produced for medium format, digital and 35 mm cameras.

The Apochromatic lens usually comprises three elements that bring light of three distinct colors to a common focus

Apochromatic designs require optical glasses with specialdispersiveproperties to achieve three color crossings. This is usually achieved using costly fluoro-crown glasses,abnormalflint glasses,and even optically transparent liquids with highly unusual dispersive properties in the thin spaces between glass elements. The temperature dependence of glass and liquidindex of refractionand dispersion must be accounted for during apochromat design to assure good optical performance over reasonable temperature ranges with only slight re-focusing. In some cases, apochromatic designs without anomalous dispersion glasses are possible.

Use in photography

Independent tests can be used to demonstrate that the "APO" designation is used rather loosely by some photographic lens manufacturers to describe thecolor accuracyof their lenses, as comparable lenses have shown superior color accuracy even though they did not carry the "APO" designation.

Also, when considering lens design, the "APO" designation is used more conservatively in astronomy-related optics (e.g. telescopes) and microscopy than in photography. For example, telescopes that are marked "APO" are specialized,fixed focal length lensesthat are optimised for infinity-like distances whereas in photography, even certain relatively low-priced general-purposezoom lensesare given the APO designation.

Often, however, apochromatic lenses used in fine cameras are not termed apochromats, Instead, they may be simply called "fluorite lenses", based on the material with anomalous partial dispersion which allowed them to be apochromatic. Such lenses began to be available to photographers in 1969, with the Canon FL-F 300mm f/5.6 telephoto lens. Fluorite has some drawbacks, for example vulnerability to sudden changes in temperature, and thus attempts were made to use substitutes, such as fluorophosphate glasses, which ameliorate, but do not completely eliminate (as compared with ordinary glass) these drawbacks.

Acrylic plastic, and, for that matter, polycarbonate, can be used to construct lenses, and their dispersion characteristics also differ from those of glass. They are, however, not normally used in the construction of apochromatic lenses, despite being much cheaper and more robust than fluorite, because the refractive index of plastics generally changes with temperature about a hundred times as much as that of glass.

See also

Focus error for four types of lens, over the visible and near infrared spectrum.

• Superachromat
• Fluorite lens
• List of telescope types